We present EON, a logic-programming language and tool that can be used to model and analyze dynamic access control systems. Our language extends Datalog with some carefully designed constructs that allow the introduction and transformation of new relations. For example, these constructs can model the creation of processes and objects, and the modification of their security labels at runtime. The information-flow properties of such systems can be analyzed by asking queries in this language. We show that query evaluation in EON can be reduced to decidable query satisfiability in a fragment of Datalog, and further, under some restrictions, to efficient query evaluation in Datalog.